We were somewhere around Barstow, on the edge of the desert, when the
drugs began to take hold. I remember "Dave Plowman (News)"
saying something like:

Although most of the UK motor industry did adopt the unified threads I'm
not so sure this applied to the US. People there still talk about AC and
AF. Of course these are often interchangeable and may just be convention.

Not always interchangeable, even if they fit.
http://www.enginehistory.org/british_fasteners.htm